1) Field of the Invention
This invention relates to a method for stabilizing properties of a ferromagnetic tunnel junction element, particularly, a method of stabilizing the resistance of an insulating film to divide a first ferromagnetic film and a second ferromagnetic film in the ferromagnetic tunnel junction element.
2) Related Art Statement
A ferromagnetic tunnel junction element has a junction structure of ferromagnetic film/insulating film/ferromagnetic film. S. Maekawa and V. Gafvert et al. have shown theoretically and experimentally in xe2x80x9cIEEE Trans. Magn.xe2x80x9d, Mag-18, 707 (1982) that the junction structure of ferromagnetic film/insulating film/ferromagnetic film exhibit a ferromagnetic tunnel effect depending on the relative angle of magnetization of both ferromagnetic films. The xe2x80x9cferromagnetic tunnel effectxe2x80x9d means the phenomenon that tunnel current to pass through a tunnel barrier layer changes depending on the relative angle of magnetization in both ferromagnetic films of an element having the junction structure of ferromagnetic film/insulating film/ferromagnetic film when a current flows in the element. The tunnel barrier layer is composed of a thin insulating film through which electron can pass with keeping spin condition, for example, a thin insulating film with a thickness of about 10 xc3x85. Since tunnel probability becomes larger if the relative angle of magnetization in both ferromagnetic films is smaller, the resistance for the current between both ferromagnetic films decreases. Conversely, since the tunnel probability becomes smaller if the relative angle of magnetization in both ferromagnetic films is larger, the resistance increases.
The tunnel barrier layer is generally composed of a thin insulating film with a thickness of about 10 xc3x85 which can be formed by oxidizing a metallic film such as Al. Since such a thin metallic film is formed by thin film forming technique such as sputtering, it has a thickness distribution corresponding to the surface-roughness of an underlayer.
Since the ferromagnetic tunnel junction element has a large magnetic field sensitivity, it has large promise as a reproducing element in super high density magnetic recording of 10 Gbit/inch2 or over. It is possible that the ferromagnetic tunnel junction element is used for another memory. For example, Kokai Publication Tokukai Hei 4-42417 (JP A 4-42417) discloses a thin film magnetic head having a ferromagnetic tunnel junction effective film which can detect the change of minute leakage magnetic flux in high sensitivity and high resolution, and enhance its reproducing sensitivity through the decrease of the generation probability of pin hole in its insulating film by narrowing its junction area.
Moreover, Kokai Publication Tokukai Hei 10-162326 (JP A 10-162326) discloses the ferromagnetic tunnel junction element, which is used for memory and external magnetic field detection, having the ferromagnetic layer in which magnetic moment does not rotate at the time of applying a magnetic field of effective intensity.
In using the above ferromagnetic tunnel junction element as a reading element in a thin film magnetic head, however, it is found that the resistance of the ferromagnetic tunnel junction element is decreased with practical operation time, and thus, the reproducing output is degraded. It is assumed that the thinner part of the tunnel barrier layer of the ferromagnetic tunnel junction element is short-circuited by the thermal energy due to a sense current at the practical operation of the reading element because the tunnel barrier layer is composed of a thin insulating film with a thickness of 10 xc3x85 and thus, has a thickness distribution corresponding to the surface-roughness of an underlayer.
It is an object of the present invention to provide a method for stabilizing properties of a ferromagnetic tunnel junction element whereby its property fluctuation can be repressed at its practical operation.
For achieving the object, this invention relates to a method for stabilizing properties of a ferromagnetic tunnel junction element comprising the step of turning the ferromagnetic tunnel junction element on electricity for a given period before its practical operation.
The above step short-circuits electrically (breaks-down softly) thinner parts of the insulating film as the tunnel barrier layer in the ferromagnetic tunnel junction element in advance, and thereby, can stabilize the resistance of the insulating film at its practical operation. Therefore, the stabilizing method of the present invention can repress the property fluctuation of the ferromagnetic tunnel junction element at the practical operation thereof. The stabilizing method decreases the output power of the ferromagnetic tunnel junction element slightly, but the decrease degree of the output power is negligible at the practical operation of the ferromagnetic tunnel junction element.
It is desired that the current to be supplied in the above stabilizing step is larger than that in the practical operation, which can reduce the stabilizing time. A current generator can supply the above current in the stabilizing process.
Moreover, the stabilizing step is preferably carried out at room temperature or over, which results in the acceleration of the stabilization and thereby, the shortening of the stabilizing time. However, it is desired that the stabilizing temperature is 300xc2x0 C. or below. If the stabilizing temperature is more than 300xc2x0 C., the ferromagnetic tunnel junction element is thermally damaged, so that its TMR (tunnel magnetoresistive effect) variation ratio is decreased extremely.
The current value to be supplied in the stabilizing step is determined on the current supplying time. That is, a large current can shorten the current supplying time and a small current requires a long current supplying time.
It is desired that the voltage to be applied to the ferromagnetic tunnel junction element is 2V or below. If a voltage more than 2V is applied, the insulating film constituting the tunnel barrier layer may be destroyed.
The stabilizing method of the present invention can be applied for stabilizing properties of at least one ferromagnetic tunnel junction element in a thin film magnetic head.
In this case, the stabilizing method may be applied for each element or whole elements at the same time. Concretely, ferromagnetic tunnel junction elements, each included in each of plural thin film magnetic heads, are connected in series, and thereafter, the above stabilizing method is carried out.
The stabilizing method of the present invention can be also applied for a wafer having many aligned thin film magnetic head elements. In this case, the wafer is improved a little. Concretely, the ferromagnetic tunnel junction elements, each included in each of the thin film magnetic head elements, are in series connected successively, and turned on electricity during a given period. To connect the ferromagnetic tunnel junction elements in series, two terminals of each element are connected successively by conductors. According to the present invention, the stabilization for the ferromagnetic tunnel junction elements can be efficiently carried out. The stabilizing step may be performed in any manufacturing step of the thin film magnetic head elements on the wafer, but is preferably done in the last manufacturing step.
The stabilizing method of the present invention can be applied for a bar shaped head aggregation having plural thin film magnetic heads in a line. The bar shaped head aggregation is produced by cutting out a wafer having plural thin film magnetic heads, and air bearing surface processing and polishing etc., are carried out for the bar shaped head aggregation. The stabilizing process may be performed in any manufacturing step of the bar shaped head aggregation, but is preferably done in the last manufacturing step after the above processing and polishing.